1. Field of the Invention
The present invention relates to an absorber pipe, especially for a parabolic collector in a solar heat collecting apparatus, which comprises a central metal pipe, a glass sleeve tube surrounding the central metal pipe to form an annular space between it and the central metal pipe, and a glass-metal transitional element arranged on a free end of the glass sleeve tube, wherein the central metal pipe and the glass-metal transitional element are connected with each other by means of at least one expansion compensating device so that they can move relative to each other in a longitudinal direction. The invention also relates to a parabolic collector using the absorber pipe and to the use of the absorber pipe.
2. Description of the Related Art
Parabolic collectors are described, for example, in “Journal of Solar Energy Engineering”, Vol. 124, p. 109 f, May 2002.
The glass-metal connection is characterized by a direct bonding of materials without additional use of adhesives (metal with glass or glass solder-low-melting glass). The boundary surface contact is achieved by pre-oxidation of the metal. The glass thus forms a permanent chemical bond with the metal oxide. Conducting the process with pretreatment and melting is decisive for quality.
Glass-metal transitional elements are described in, e.g., U.S. Pat. No. 1,292,466, U.S. Pat. No. 1,293,441 or U.S. Pat. No. 6,324,870 B1. This sort of glass-metal transitional element has a tapering metal end section, which is surrounded by a glass body, and usually has large linear thermal expansion differences between the glass and metal. The length difference is absorbed in the very thin metal.
In the so-called fitted glass-metal transitional element, like those e.g. in X-ray tubes, the expansion coefficients for the glass and metal are equal in size. Also this transitional element achieves a good bond by pre-oxidation.
The glass-metal transitional elements can be glass-sealed directly or connected by glass solder.
The absorber pipe usually comprises an inner radiation-absorbing coated steel pipe and a surrounding glass tubular sleeve. The individual absorber pipes are about 4 m long and are assembled to form a solar field loop with a length of up to 800 m. The solar radiation is concentrated on the pipe by a mirror arranged behind the pipe. A glass-metal transitional member connects the glass tube and metal pipe with each other in a gas-tight manner. The intervening space between the metal pipe and the glass tube is evacuated to minimize the heat loss and to raise the energy yield. Expansion compensation is required between the metal pipe and tubular glass sleeve, because of the different thermal expansion coefficients of metal and glass and because of the great difference in heating in operation in which the metal pipe reaches about 400° C. and the glass tube only 100° C. This expansion compensation is generally provided by a metal folding bellows, as described in “Journal of Solar Energy Engineering”, May 2002, Vol 124, p. 109 and following, page 115.
In the currently known structures the glass-metal transitional element and the folding bellows are arranged linearly next to each other. This has the result that a significant part of the pipe surface of about 2 to 3% cannot be used as a collecting surface for the solar radiation. This leads to a reduction in the optical efficiency.
The solar radiation reaching the pipe directly makes the protection of the thermally stressed glass-metal transitional element from heat required.
Additional screens provide this protection, however they do not sufficiently screen off the radiation reaching the sleeve tube in slanting light, which enters the sleeve tube and is unsuitable for the absorber. Because of that a breakdown of the glass-metal connection occurs on the north side of the absorber pipe. The result is loss of vacuum and because of that the efficiency of the absorber tube is limited. The exchange of individual absorber tubes is extremely expensive because the entire solar field loop must be idled or put out of operation for that reason. Usually these features are avoided and an overall power reduction from the entire solar field is accepted by running the absorber pipe with a rate of more than 2% per year.
Different solutions with a sliding bearing between the glass tube and the sleeve tube are currently proposed to reduce the shadow effect and to maximize the aperture surface.
A compensating device is known from DE 100 36 746 A1, which has an axially slidable sealing device, which is attached to one of the pipes, while the other pipe is slidable axially on the sealing device. The sealing device, for example, can be shrunk on the metal pipe and can be slidable relative to the tubular glass sleeve.
The other sealing device according to DE 100 36 746 A1, which is arranged on the pipe ends, has a circular groove for receiving an end of the sleeve tube so that it is sealed in the circular groove. The sealing device is held fixed on the central pipe in this case. A chamber, which is bounded by the sealing device and this end of the tubular sleeve, is provided in the circular groove. This chamber is designed for receiving a liquid acted on with pressure. This arrangement is expensive and does not operate reliably, so that loss of vacuum after a short time must be considered. However if no vacuum is present, the absorbing coating degrades at high temperature under normal atmospheric pressure substantially more rapidly than under vacuum.
A warm water pipe collector is known from U.S. Pat. No. 4,231,353, which is provided only for operating temperatures of 70° C. A metal plate, which is glued to the front face of the tubular glass sleeve by an adhesive, is arranged between the tubular glass sleeve and the central metal pipe. This arrangement has no glass-metal transitional element. The expansion compensation device, which guarantees a larger shift or displacement of the pipes in the longitudinal direction relative to each other, is not present. In all cases the cover provided with a circular bead can take small length differences, however undesirable radial forces are exerted on the glass edge. This collector pipe is thus basically unsuitable for use in a parabolic collector.